19J6 replacement

[bacon665]

Hi, i'm looking at building Peter Millets starving student amplifier,

But im interested in using a matched pair of GE 12V6GTs.
I was wondering if anyone had any idea on how to do such a swap, do i need to just conect the two grids together and lower the 19volts to 11/12 or is this swap a bad idea?

Thanx

[ilimzn]

12V6 is a completely different tube (for one thing it's a single pentode, not a double triode!) and would require a redesign of the whole amp, even triode connected - as it's gain and transconductance are completely different (about 1/5th gain, and 1/3 transconductance). In addition, the heater uses signifficantly more current, and in this design it doubles as the MOSFET's source resistor. It would be possible to design something similar around the 12V6 but pretty much every single component would have a different value, except the power supply cap!

[ethermion]

Also note that 19J6s are cheaper than a pack of Camels. $4 a pop here: http://www.esrcvacuumtubes.com/vacuu...3CW4-90c1.html

[dsavitsk]

These have gotten nearly impossible to find. Best place for info is over at head-fi. My suggestion for an alternative would be the 12au7 -- runs OK at the low voltages specified, and heater current is the same. You'll be dropping an extra 7V across the fets, so you'll need to adjust the bias resistor accordingly and be sure the heatsinks are up to snuff.

[ethermion]

http://www.fairradio.com/ has 'em for $4.50. http://www.vacuumtubes.net/ has 'em for $4.00.
http://www.vacuumtubes.com/price8.html has 'em for $10.00

[bacon665]

I inherited an old RCA TV Repair case that was filled with two manuals and about 1000 tubes of assorted types. i dont really see why it uses two dual triodes connected as a single instead of two triodes.
The closest things i have to 19v
(aside from some magnavox full wave rectifiers)
are:
4 12BH7A (1 matched pair)
1 12AT6
1 12AT7
3 12AX7A (matched)
1 12AX7WA
and 1 15H36

I was under the assumption that it was possible to place a capacitor parralel to ground and a resistor in series before the heater to lower the volatge to 12.

As dsavitsk said these are pretty hard to find.
Out of the handful of sites that sell them most are out of stock, as with any fear i can easily see the 19J6's price skyrocketing way out of my budget.

[dsavitsk]

Originally posted by ethermion
http://www.fairradio.com/ has 'em for $4.50. http://www.vacuumtubes.net/ has 'em for $4.00.
http://www.vacuumtubes.com/price8.html has 'em for $10.00

Try ordering some and see what shows up.

[ilimzn]

Originally posted by bacon665
I inherited an old RCA TV Repair case that was filled with two manuals and about 1000 tubes of assorted types.

Would you care to list anything that has a more than 6V heater, including single tridoes and pentodes or BPTs? Maybe something can be found that is suitable...

i dont really see why it uses two dual triodes connected as a single instead of two triodes.

That would be ebcause you don't seem tio understand how it really works
The 19J6 (6J6 with 150mA heater) has two triodes with a common cathode. In order to get decent current capability and gm at such low voltage power supply, both triodes are paralelled.
The reason why a 19V heater version is used has more to do with the heater being a 15mA type - it doubles up as a source resistor for the MOSFET source follower. In other words, the output signal is developed across the heater!

The closest things i have to 19v
(aside from some magnavox full wave rectifiers)
are:
4 12BH7A (1 matched pair)
1 12AT6
1 12AT7
3 12AX7A (matched)
1 12AX7WA
and 1 15H36

The 12BH7A has a 300mA heater, so twice the current will flow through the MOSFET, which means twice the ehat is generated, which means a bigger heatsink. But, it should be doable although low voltage performance is not s good as the *J6...
12AT7 and 12AX7 have the right heater, but can't work well at such low voltages. 12AT6 may even work at 48V but only at a fraction of a mA, insufficient to drive a MOSFET even in a source follower configuration -

I was under the assumption that it was possible to place a capacitor parralel to ground and a resistor in series before the heater to lower the volatge to 12.

You can place a resistor in series with the heater to add the extra 7V, assuming a 150mA heater, the resistor would be 47 ohms at least 3W. NO capacitor - as I said, the output voltage is developed over the heater (in this case incl. resistor).

As dsavitsk said, 12AU7 with sections in parallel may work, but the cathode and anode resistors have to be recalculated.

[Coreyk78]

I've built 2 starving students and I can definitely sympathize with how difficult it is to find any 19J6 out there, I have 10, but I'm keeping those as spares. One alternative tube that I read about in the thread over at head-fi is the 12SR7/12SW7. Its an octal so the socket pin connections have to be changed and obviously some resistor values change but its all spelled out in the post there.

I ordered a handful of 12SR7 and a pair of 12SR7GT for myself to try this tube out since I already have the rest of the parts and compare it to the 19J6 version.

Also thanks dsavitsk, I didn't know the 12AU7 was a possibility, I have a bunch of those already, hmmm...

Originally posted by Logistic on head-fi:

--------------------------------------------------------------------------------

Hi everyone,

This has been in the works for a while but I finally found some time to put it together. I wanted to modify the starving student amp using an alternative tube to the scarce 19J6's. I finally settled on the 12SR7 and 12SW7 which are only $1 each at vacuumtubes.net. Perfect for the starving student. The caveat is that they are metal tubes and some people may prefer glass tubes. In that case the 12SR7GT is the glass tube version of the 12SR7. It is a few dollars more per tube but still cheap.


Here is the result, the metal starving student:


If you want to build this version, you have to make some small changes -

R1/R7 - 44k
R5/R11 - 4K
R2/R8 - 330K
R4/R10 - 180K

You will have to use an octal socket, and here are the conversions from the seven pin socket in the original circuit diagram:

1,2 -> 6
5,6 -> 2
3,4 -> 7,8
7->3

You will also have to ground pin 1 which is for the metal casing. It is probably a good idea to ground pin 1 with a separate path to ground from the heaters (pins 7,8). That way if the ground connection for the heaters breaks for whatever reason you don't see that voltage on the tubes.

Using 2" heatsinks is also recommended since the heaters are running at 12V instead of 19V.
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[bacon665]

Thanks Coreyk78 =)

I'm going to order some 12SW7's tonight.